Once an annotation type is defined, you can use
it to annotate declarations. An annotation is a
special kind of modifier, and can be used anywhere
that other modifiers (such as public, static, or
final) can be used.
@ANNOTATION 是一个 modifier，就跟 public、static，final 一样。
通产放在其它 modifier 前面
By convention, annotations precede other modifiers.
Annotations consist of an
at-sign (@) followed by an annotation type and a
parenthesized list of element-value pairs. The
values must be compile-time constants. Here is a
method declaration with an annotation
corresponding to the annotation type declared
above: